In commercial hybrid breeding, cytoplasmic male sterility (CMS) is used to block self-pollination of plants. In turn, Restorer-of-fertility (Rf) genes are applied to supress CMS and restore self-pollination.
Most restorer genes in crops encode RNA-binding pentatricopeptide repeat (PPR) proteins. Plants have hundreds of PPR proteins but only a few act as Restorer-of-fertility and these define a clade referred to as ‘Restorer‑of‑fertility‑like’ PPRs (RFL-PPRs). Recently, several candidates for restorer genes have been found to encode members of the mitochondrial transcription termination factor (mTERF) family. No sequence similarity between mTERF and PPR proteins exists, however, like PPRs, mTERFs function in RNA-associated processes and can directly bind to RNA. Currently, mTERFs remain a largely understudied group of organellar sequence-specific RNA-binding proteins in plants.
Our studies have shown that a group of mTERF genes share several genomic features with the RFL-PPR clade. We demonstrated that, as for RFL-PPRs, one clade of mTERF genes is highly expanded in plants, especially cereals and that they are organised into clusters adjacent to or intermixed with RFL-PPR clusters in the genome. Similarly to RFL-PPRs, clustered mTERFs showed extreme levels of copy-number and structural variation within and between species and illustrate the evolutionary mechanisms at work. Our analyses revealed that the location of mTERF clusters overlaps with mapped Rf loci in wheat and rye. Based on these discoveries we named this clade RFL-mTERFs.
Future studies will aim to characterise the newly discovered RFL-mTERF clade and understand its contribution to fertility restoration in plants.